This invention relates to thermographic transcription apparatus, and more particularly to thermographic stylus means having inherent pressure control.
In a thermographic apparatus a heated stylus or scribing element is in contact with a writing platen covered by a paper having a blushed lacquer finish. As the stylus moves along the paper, it produces opacity or color change on the finish of the paper, tracing its path over the paper.
While these systems generally are adequate to transcribe the voltage signals, the quality of the transcription, either in line width or opacity, may deteriorate with rapidly changing signals or high writing speed. More particularly, for a given temperature and pressure, the more rapid the time rate of change which the signal experiences, the more variability will occur in the quality of the transcribed signal. That is, if the input voltage signal changes rapidly and has a high transient content, drag between the stylus and the paper may limit the responsiveness of the stylus, thereby causing some or all high frequency components of the voltage signal to be lost in the transcription process. Conversely, if stylus pressure is adjusted to be sufficiently light that drag problems are avoided, a poorer quality of transcribed signal results, an unfortunate consequence for signals having lower rates of change and therefore not experiencing problems of drag.
It is accordingly a primary object of the present invention to provide a self actuated thermographic stylus in which the pressure exerted is adequate to produce a high quality transcription for low signal rates, but which is adjustable in proportion to signal changes or writing speed.